The men argued over the right approach to dealing with whistleblowers on January 23, 2014.

Mr McGuinness questioned why Mr Callinan would not, as the Garda's Accounting Officer, ask somebody to telephone the whistleblower and call him or her in for a meeting.

He added: "I am not telling Mr Callinan how to do his job, as I would not dare do that, but this would appear to be the commonsensical, simple way to deal with the matter."

The then Garda Commissioner said the door was "always open for anybody".

"The chairman is almost challenging me in my attitude towards these people. I have repeatedly told him and this committee that, if anyone observes any wrongdoing at any time, the proper action for those people as responsible members of An Garda Síochána is to report it. The chairman has heard me say this several times," Mr Callinan said.